In optical telecommunications, as well as in other fields where light pulses are used (e.g., in laser fusion), the need for optical pulse shaping is well recognized. For example, in fiber-optical communications, as a pulse tends to be distorted due to dispersion in the course of transmission over an optical fiber, pulse shaping may be used to advantage at a transmitter or at a receiver. Indeed, compensation for optical dispersion is a principal motive for pulse shaping.
Optical dispersion can be understood in terms of frequency-dependent propagation velocities of sinusoidal waveforms constituting a signal. In typical transmission media, dispersion is either directly or indirectly related to frequency, dispersion being termed "normal" in the case of a medium in which higher-frequency waves travel more slowly, and "anomalous" in the opposite case. Typically also, dispersion is nonlinearly related to frequency, so that it is meaningful to consider higher-order terms of a functional relationship between dispersion and frequency, e.g., second- and third-order terms. Higher-order dispersion is particularly significant in the transmission of ultrafast (subpicosecond, terabit) optical signals. Such signals are preferred in so-called code-division multiple-access communications, a field which is under active current development; see, e.g., U.S. Pat. No. 4,866,699, issued Sep. 12, 1989 to C. A. Brackett et al.
One class of pulse-shaping methods and devices, disclosed in U.S. Pat. No. 4,655,547, issued Apr. 7, 1987 to J. P. Heritage et al., is predicated on spatial dispersion of frequency components of a signal, combined with the use of a spatial amplitude and/or phase mask. Motivation for an aspect of the invention described below stems from the desire to provide pulse-shaping means which are particularly easy to implement and which can be fabricated by simple mechanical assembly.